Model to hardware correlation measurements on all packaging levels are essential in today's development process of high performance computers. Different measurement techniques in time and frequency domain require different measurement set-ups and test site designs. One demand for the test site is to be equivalent to the product. Therefore, transmission lines on a chip need to be measured in the product like power and ground wiring distributed in all metal layers on chip. In addition it is not only of interest to measure a single transmission line but also with a product like wiring channel utilization. This is essential to real signal coupling behaviour on the chip and the effect of metal layers between top metal layers and the semi conducting substrate.
A known measurement technique is the so-called S-parameter measurements, see Zinke/Brunswig, “Lehrbuch der Hochfrequenztechnik”, Springer-Verlag, 1989, pp. 170-189. S-parameters are reflection and transmission coefficients of an n-port network. The equivalent for a single transmission line e.g. is a two port network characterized by a 2×2 S-parameter matrix.
A two-port network is described by the relationship
      (                                        b            1                                                            b            2                                )    =            (                                                  S              11                                                          S              12                                                                          S              21                                                          S              22                                          )        ·          (                                                  a              1                                                                          a              2                                          )      wherein S11, S22, S12 and S21 are the S-parameters, namely                S11=Input reflection coefficient with the output port terminated by a matched load,        S22=Output reflection coefficient with the input terminated by a matched load,        S12=Reverse transmission (insertion) gain with the input port terminated in a matched load,        S21=Forward transmission (insertion) gain with the output port terminated in a matched load, and the variables a1, a2 and b1, b2 are complex voltage waves incident on and reflected from the first and second port of the two-port network.        
In the present case the S-parameter measurements is an advantageously measurement technique because the S-parameter are easier to measure and work with at high frequencies than other kinds of parameters.
Furthermore, different methods are well-known in the state of the art to extract other characteristic frequency dependent line parameters, such as characteristic impedance Z(f) or propagation constant γ(f) etc., from the S-parameter measurements, so that these parameters can be easily obtained from the S-parameter measurements, Thomas-Michael Winkel, Lohit Sagar Dutta, Hartmut Grabinski, “An Accurate Determination of the Characteristic Impedance of Lossy Lines on Chips Based on High Frequency S-Parameter Measurements”, IEEE Multi-Chip Module Conference MCMC'96, pp. 190-195, February 1996, Thomas-Michael Winkel, “Untersuchung der Kopplung zwischen Leitungen auf Silizium-Substraten unterschiedlicher Leitfähigkeit unter Verwendung breibandiger Messungen”, Ph D. Thesis, University of Hannover, November 1997, pp. 50-77.
A requirement for the high frequency S-parameter measurements in this case is that the transmission lines are not connected to any active device on chip.
A special requirement for the high frequency S-parameter measurements is that the signal line structure, including the voltage and ground lines, has to be designed in a way to ensure that only one signal mode can be propagated along the signal line structure. Different signal modes can propagate along a transmission line in presence of one or more adjacent signal lines or if the transmission line is referenced to more then just one reference plane or line.
As evidenced from the forgoing discussion, it is desirable to provide a measurement system for determining the S-parameters as a function of the frequency of an electrical signal line which does not suffer from the above-noted drawbacks and leads to a significant gain of the measurement bandwidth.